Photoresponsive peptide and polypeptide systems. Part 7. Reversible chiral photochromism and solubility change of azo aromatic L-lysine related compounds

Abstract

Disubstituted azo aromatic L- and D-lysine homologues with one to four methylene groups in the side chain, and related compounds, have been synthesized. The photochemical properties of the azo aromatic lysine series have been studied from both achiral and chiral standpoints. On irradiation at different wavelengths, the azo aromatic lysine series change their solubilities achirally in certain solvents, being soluble under u.v. light (cis) and precipitating out under visible light (trans). The dipole moments of the solvents used are considered to be important in this solubility change. The chiral photochemical properties of the above lysine related compounds have been examined by absorption and circular dichroism (c.d.) spectroscopy. The whole c.d. spectrum exhibits reversible photochromism on irradiation with light, owing to the trans-cis photoisomerization of the azo aromatic moieties. The exciton chirality method has been successfully applied in order to demonstrate the origin of the dichroic bands in the 260–390 nm wavelength region which exhibit coupling of the transition dipole moment due to the transπ-π* transition of the azo aromatic chromophore under an asymmetric Cα carbon environment.